Spiro compounds spiropentanes

A second mole gives spiropentanes. In fact, any size ring with an exocyclic double bond can be converted by a carbene to a spiro compound. ... 

Spiro substitution is common in both natural products and in molecules so far encountered only in the laboratory. Note in Figure 5.48 how poorly a two-dimensional picture represents the real shape of such molecules. Even the simplest spirocyclic compound, spiropentane, is badly served by the flat page. The central carbon is approximately tetrahedral, and we really must use wedges to give a three-dimensional feel to this kind of molecule. 

Thermolysis of Spiro-compounds. It is already known that pyrolysis of spiro-pentane gives products whose relative yields depend upon the pressure. Flowers and Gibbons have extended earlier work and shown that RRKM theory accounts for the pressure dependence of the reaction from lO Torr to 1 atm using either of two possible mechanisms. One mechanism involves intervention of a biradical intermediate, the other involves direct transformation of spiropentane into methylenecyclobutane, ethylene, and keten (and... 

Reductive dehalogenation is an efficient method of synthesis of cyclopropanes spiroannulated to five- and higher-membered carbocycles (i.e. compounds in which spiroannulation does not result in accumulation of extra strain) . The required gem-(dihalomethyl)cycloalkanes are usually prepared by halogenation of the precursor diols (equation 1). The cyclization is most efficiently accomplished in the Zn-alcohol-water system . For example, spiro[2.5]octane 7 was prepared in 91% yield using this procedure. This method is useful even for a one-step preparation of bis-spirocyclopropyl compounds as exemplified in equation 2. However, the application of the reductive dehalogenation method to the synthesis of more strained SPC (i.e. spirohexane or spiropentane) often leads to rearranged products. For example, methylenecyclopentane was the only product obtained from bis(bromomethyl)cyclobutane (equation 3) ... 

Cyclopent-2-enone rings can be easily obtained under mild conditions from a,a-dibromoketones and enamines in the presence of iron carbonyls Spiro-annelation with S-cyclopropylids provides easy access to spiropentane derivatives and similar compounds . Ring expansion from 5- to 7-membered rings has been efficiently performed with cyclopropenes The transformation of nerolidol to the tricyclic a-cedrene has been achieved by a remarkable stepwise cyclization process... 

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